Improving the field-emission properties of carbon nanotubes by magnetically controlled nickel-electroplating treatment

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1 Improving the field-emission properties of carbon nanotubes by magnetically controlled nickel-electroplating treatment Zheng Long-Wu( ), Hu Li-Qin( ), Xiao Xiao-Jing( ), Yang Fan( ), Lin He( ), and Guo Tai-Liang( ) Institute of Optoelectronic Display Technology, School of Physics and Information Engineering, Fuzhou University, Fuzhou , China (Received 19 May 2011; revised manuscript received 28 June 2011) A novel magnetically controlled Ni-plating method has been developed to improve the field-emission properties of carbon nanotubes (CNTs). The effect of the magnetic field and Ni-electroplating on CNT field-emission properties was investigated, and the results are demonstrated using scanning electron microscopy, J E and the duration test. After treatment, the turn-on electric field declines from 1.55 to 0.91 V/µm at an emission current density of 100 µa/cm 2, and the emission current density increases from to 0.34 ma/cm 2 at an electric field of 1.0 V/µm. Both the brightness and uniformity of the CNT emission performance are improved after treatment. Keywords: carbon nanotubes, magnetic field, field emission, Ni-electroplate PACS: Fd, rh, h DOI: / /20/12/ Introduction Due to special geometrical morphology, and properties including high aspect ratio, small tip radii of curvature, high current capacity, large field enhancement factor and high electrical conductivity, carbon nanotubes (CNTs) are ideal materials for efficient field emitters. [1 4] In recent years, many research studies have been performed on CNTs as field emission (FE) cathode materials. [5 8] In general, CNT FE cathodes could be made by different methods such as chemical vapour deposition (CVD), [9 11] screen-printing, [12,13] electrophoresis [14 16] and the spray method. [17,18] The direct growth of CNT FE cathodes using CVD usually has low-throughput, and the high growth temperature would limit the choice of substrate materials. The other methods can be carried out under low temperature and have the advantage of being low cost, simple processes. However, these methods usually have the problem of no CNT emitters vertical to the surface, which leads to the poor emission properties of CNTs. There are many post-treatments used to improve the FE characteristics, such as adhesive taping, [19] soft rubber rolling, [20] focused ion beams, [21] plasma treatment, [22] and elastomers. [23] However, adhesive taping and soft rubber rolling lead to contaminants, and are limited to the simple diode-type cathode structure. Focused ion beam and plasma treatments require expensive instruments and complex proceedings, and the elastomer treatment leads to a residual elastomer. In this paper, a simple and effective posttreatment based on magnetically controlled Nielectroplating to erect the CNT emitters is presented. Not only does our method prevent residual pollution, but it also makes the CNT upright. As a result, the CNT emitters treated by the method show higher FE performance than those that were untreated. 2. Experiment First, multi-walled CNTs (4g) with a diameter of 8 15 nm, a length of about 30 µm and a purity greater than 95% were dispersed in 1-L isopropyl alcohol solution containing 10 g/l dissolved polyvinylpyrrolidone (K30) by the ultrasonic cell crusher (model: SCIENT2-IID) at power equal to 2% for 10 min. Second, the preparation of the electrophoresis suspension, Project supported by the National High Technology Research and Development Program of China (Grant No. 2008AA03A313) and the Natural Science Foundation of Fujian Province of China (Grant No. 2009J05145). Corresponding author. gtl fzu@yahoo.com.cn 2011 Chinese Physical Society and IOP Publishing Ltd

2 10 ml of the 4 g/l multi-walled carbon nanotubes (MWCNTs) suspension and 0.2 g Mg(NO 3 ) 2 6H 2 O, was added into the 1-L isopropyl alcohol solution for 60 min ultrasonic treatment. After that, the electrophoresis process was carried out. The graphite plate was used as the anode and the glass plate covered by Cr Cu Cr electrodes (produced by magnetic control sputter coaters) as the cathode; the distance between the cathode and the anode was 5 mm, and the voltage in the process was 12 V for 5 min. Third, the magnetically controlled Ni-electroplating process was carried out as depicted in Fig. 1(a): the electrophoretic prepared CNT electrodes were the cathode and the Ni plate was the anode; the distance between the cathode and the anode was 5 cm, and the magnitude of the square magnet beside the vessel was 1.2 T. The process was performed in a nickel sulphate bath with the following composition: 150- g/l NiSO 4 7H 2 O, 30-g/L NiCl 2 6H 2 O, 40-g/L H 3 BO 3 and 0.1-g/L C12H 25 SO 4 Na. The treatment was carried out at 60 C for 30 s at the current density of 1 ma/cm 2 with a ph value of about 5.4. Fourth, the cathode was dried in an oven at 200 C for 20 min and sintered in an oven filled with 99.99% nitrogen at 500 C for 30 min to remove the organic residue (such as the K30 in the electrophoresis process), and then cooled down to room temperature. Fifth, the Nielectroplating CNT cathode was treated by the square magnet as depicted in Fig. 2 (the third, fourth and fifth steps of the experiment comprised the magnetically controlled Ni-electroplating treatment). The morphologies of the CNT cathode were characterized by scanning electron microscope (SEM, S-3000 HITACH). The FE images of the samples were measured in conventional diode configuration, with the gap between the cathode and the anode 1100 µm, the pressure Pa, the cathode area 5 cm 8 cm, and the DC voltage 1300 V. Indium tin oxide (ITO) glasses coated with green-phosphor were used as the anode. 3. Results and discussion Figure 1 shows the magnetically controlled Nielectroplating process. After the electrophoresis process, the cathode was covered by massive CNT. During the magnetically controlled Ni-electroplating process, the Ni plate was the anode and the cathode was the CNT-covered electrode. With a supply of current density at 1 ma/cm 2 for 30 s, the Ni particles travelled across the suspension to the cathode, went through the massive CNTs and attached onto the metal cathode. [24] Since the square magnet was on the left side, the travelling Ni particles were magnetically aligned on the cathode surface. According to the theory proposed by Fujiwara et al., [25] when an external magnetic field is applied, the CNTs on the cathode are magnetized, as the deposition of the Ni particles on the cathode surface. As depicted in the enlarged view of the cathode in Fig. 1(b), the CNT attached to the cathode surface is polarized to negative, while the CNT tips are polarized to positive, which contributes to the magnetic alignment process of the CNTs. Fig. 1. (a) The magnetically controlled Ni-electroplating process, (b) an enlarged view of the cathode. After the magnetically controlled Nielectroplating process, the CNT cathode was dried in an oven at 200 C for 20 min and sintered in an oven filled with 99.99% nitrogen at 500 C for 30 min to remove organic residue, and then cooled down to room temperature. The CNT cathode was then placed beside the square magnet (magnitude 1.2 T) with a distance of 5 mm for 10 min, as depicted in Fig. 2. According to the theory proposed by Sharma et

3 ln(j E 2 ) Chin. Phys. B Vol. 20, No. 12 (2011) al., [26] with the supply of magnetic field, the nanotube tips, which had been polarized, became magnetized to saturation, behaved as a magnetic dipole, erected themselves from the surface, and then completed the alignment process. The electrical conduction nature of these aligned CNTs is dramatically improved in comparison with the randomly lying CNTs. Fig. 2. The CNT cathode treated by the square magnet. In Fig. 3(a), almost all the electrophoretic deposited CNTs are laid on the surface, and it is difficult to find any MWCNTs vertical to the substrate. While in Fig. 3(b), there are obviously CNTs vertical to the surface due to the magnetically controlled Ni CNT alignment process, and Ni particles accumulated on the CNT tip, body and substrate, which contribute to the magnetic alignment process. From Fig. 4, the improvement in CNT FE characteristics before and after magnetically controlled Ni-electroplating treatment is clear: the turnon electric field declines from 1.55 to 0.91 V/µm at an emission current density 100 µa/cm 2 ; and the emission current density increases from to 0.34 ma/cm 2 at the electric field 1.0 V/µm. The FE of the treated and untreated specimens qualitatively follows the conventional Fowler Nordhein (FN) theory, observed in Fig. 4 (inset). According to the FN equation, the emission current density is J=A(βE) 2 exp( Bϕ 3/2 /βe), where A and B are constants, E is the applied electric field, ϕ is the work function and β is the field enhancement factor. Under the assumption that the work function ϕ of the CNTs is equal to that of graphite (5.0 ev), the field enhancement factor β after the magnetically controlled Ni-electroplating treatment is 3590, higher than the 1630 before the treatment treated untreated J mascm E E/VSmm -1 Fig. 4. Field-emission characteristics of the CNT emitters: I V curve and FN plot (inset). Fig. 3. The morphologies of the CNTs before (a) and after (b) magnetically controlled Ni-electroplating treatment. The morphologies of the CNTs on the cathode before and after the magnetically controlled Nielectroplating treatment were measured by SEM (S HITACH), and are shown in Fig. 3(a) and (b). The FE images (both captured at exposure time 1/60 s and ISO:100) before and after the magnetically controlled Ni-electroplating treatment are shown in Fig. 5(a) and (b). It is clear that the treated one is brighter and more uniform than the untreated one. We assume three reasons for the improvement in CNT FE properties by magnetically controlled Nielectroplating treatment. First, there are many CNTs

4 vertical to the cathode surface and so the FE characteristics will be greatly improved, [27] according to the theory proposed by Wang et al. Second, according to electro magneto theory, the electric current gives rise to an electromagnetic field; the reason for the magnetization is the acceleration and conformity of the molecular current. After magnetic field alignment treatment, the electrons in the CNT material become more active and reach a higher Fermi level, which makes the FE easier than in the untreated ones. Third, on the one hand the electrons moving from the substrate to the vacuum follow the double-barrier model as shown in Fig. 6. The FE electrons tunnel through two barriers: first, electrons tunnel through barrier I between the CNTs and the substrate into the CNTs, then they tunnel through barrier II and are emmitted into the vacuum from the CNTs. On the other hand, the Ni particles electroplated on the substrate have good electrical conductivity, which contributes towards reducing barrier I. Therefore, with the Ni particles connecting the substrate and the CNTs, barrier I will be reduced and it is easier for the electrons to tunnel through barrier I between the CNTs and the substrate, which contributes towards an improvement in FE properties. E FM metel barrier I V junction q nanotubes barrier II E FCNT vacuum Fig. 6. The band structure of the double-barrier model for the field emission of the CNT film. E FM and E FCNT are the Fermi level of the substrate metal and the CNTs, respectively. V junction is the voltage dropped on the substrate CNT junction. In order to compare the degradation rate of the two cases, a lifetime measurement was performed for 20 h with a DC voltage of 1300 V. The initial current density of the two samples is about and ma/cm 2, respectively. As shown in Fig. 7(a), after 20 h the current density of the CNTs prepared by magnetically controlled Ni-electroplating treatment decreased to 75% of the initial current, whereas in Fig. 7(b) that of the CNTs without treatment dramatically decreased to 40% of the initial current, which means that the CNT emitters after magnetically controlled Ni-electroplating treatment had a longer duration than those that were untreated. Generally, Fig. 5. Field-emission images of two specimens: before (a) and after (b) magnetically controlled Ni-electroplating treatment. Fig. 7. Field-emission stability test of the two cases: treated (a) and untreated (b) by magnetically controlled Ni-electroplating treatment

5 the emission current of CNTs decreases when residual gas molecules are adsorbed by Joule heating under a strong electric field. [28] The magnetically controlled Ni-electroplating treatment is advantageous at high vacuum, since there are no organic vehicles as the primary out-gassing sources in the electroplating suspension, unlike the electrophoresis suspension containing polyvinylpyrrolidone (K30). It is therefore confirmed that the above magnetically controlled Nielectroplating treatment is facile and effective. 4. Conclusion This work demonstrates that most CNTs on the cathode are magnetically erected to the surface by the magnetically controlled Ni-electroplating treatment. The FE properties of the CNTs are improved, the brightness and uniformity of the CNT diode configuration are promoted. Magnetically controlled Nielectroplating treatment is facile and effective in FE display applications. References [1] Iijima S 1991 Nature [2] Li Y, Zhu C and Liu X 2002 Diamond Relat. Mater [3] Shi Y, Zhu C C, Wang Q and Li X 2003 Diamond Relat. Mater [4] Li J, Lei W, Zhang X, Zhou X, Wang Q, Zhang Y and Wang B 2003 Appl. Surf. Sci [5] Dai J F, Mu X W, Qiao X W, Chen X T and Wang J H 2010 Chin. Phys. B [6] Bai X, Zhang G M, Wang M S, Zhang Z X, Yu J, Zhao X Y, Guo D Z and Xue Z Q 2009 Chin. Phys. B [7] Zhang Y, Cao J X and Yang W 2008 Chin. Phys. B [8] Song L, Liu S, Zhang G M, Liu L F, Ma W J, Liu D F, Zhao X W, Luo S D, Zhang Z X, Xiang Y J, Shen J, Zhou J J, Wang G and Zhou W Y 2006 Chin. Phys [9] Jung M, Eun K Y, Lee J K, Baik Y J, Lee K R and Park J W 2001 Diamond Relat. Mater [10] Lee C J, Park J, Huh Y and Lee J Y 2001 Chem. Phys. Lett [11] Wang B B, Gu C Z, Dou Y, Wang G J, Li H J and Zhu M K 2003 Chin. Phys [12] Shi Y S, Zhu C C, Wang Q K and Li X 2003 Diamond Relat. Mater [13] Kwo J L, Yokoyama M, Wang W C, Chuang F Y and Lin I N 2000 Diamond Relat. Mater [14] Wang L L, Chen Y W, Chen T, Que W X and Sun Z 2007 Mater. Lett [15] Choi W B, Jin Y W, Kim H Y, Lee S J, Yun M J, Kang J H, Choi Y S, Park N S, Lee N S and Kim J M 2001 Appl. Phys. Lett [16] Choi Y S, Choi G S and Kim D J 2006 Electrochem. Solid State Lett. 9 G107 [17] Lim S C, Choi H K, Jeong H J, Song Y I, Kim G Y, Jung K T and Lee Y H 2006 Carbon [18] Jeong H J, Choi H K, Kim G Y, Song Y I, Tong Y, Lim S C and Lee Y H 2006 Carbon [19] Vink T J, Gillied M, Kriege J C and van de Laar H W J J 2003 Appl. Phys. Lett [20] Kim Y C, Sohn K H, Cho Y M and Yoo E H 2004 Appl. Phys. Lett [21] Sawada A, Iriguchi M, Zhao W J, Ochiai C, Takai M and Vac J 2003 Sci. Technol. B [22] Lee K, Lim S C, Cho Y C and Lee Y H 2008 Appl. Phys. Lett [23] Lee H J, Lee Y D, Cho W S and Ju B K 2006 Appl. Hhys. Lett [24] Ning Z H, He Y D and Gao W 2008 Surf. Coating Technol [25] Fujiwara M, Oki E, Hamada M and Tanimoto Y 2001 J. Phys. Chem [26] Sharma A, Tripathi B and Vijay Y K 2010 J. Memb. Sci [27] Wang X Q, Wang M and Li Z H 2005 Ultramicroscopy [28] Hata K, Takakura A and Saito Y 2001 Surf. Sci